In terminal oriented computer systems and computer network systems it is quite frequently desirable to utilize preexisting telephone lines and or facilities for connecting satellite computer and/or input-output terminals to a central computing facility. The utilization of existing telephone lines saves significant amounts of money contrasted with the expense of installing separate lines for connecting said terminals to the parent system. This is true when the communication system must connect, for example, bank branches located throughout a large metropolitan area to some central computing facility or even interconnect typewriter-like terminals within a single office building to a central computer room.
With such systems it is, of course, possible for the terminal operator to physically ring-up the central computer system over a telephone line and personally make the connection when an indication is received that a line is available. Such a system however is not particularly efficient in terms of operator time and especially where there is a great deal of traffic, i.e., where the computer input lines are being used a high percentage of the time, with a consequent small percentage of time available for a new job to connect to the system. It would obviously be quite advantageous to have a system wherein the terminal attempts to connect to telephone line and is able to automatically determine the line status. This could result for example in temporarily hanging up and trying later, or beginning the communication of a message to the central system, if the connection is successfully made.
It is also a desirable feature to be able to acoustically couple to telephone lines so that in essence a more or less conventional telephone set can be used and data fed into the telephone line or received from the telephone line utilizing acoustic means with said set. With acoustic coupling it is not necessary to modify either the computer terminal or the telephone line itself as any data to be transmitted in either direction is placed on an appropriate acoustic carrier capable of being transmitted within the bandwidth capabilities of the line without significant attention. However, to utilize an unmodified conventional voice line in a completely automatic (without human intervention) fashion it is necessary to be able to automatically detect the line status signals which the telephone company conventionally uses.